1. Field of Invention
This invention relates to modulators for optical fiber communication and photonics analog fiber links systems. More specifically, it relates to a new class of optical modulators that provide ultra-linear signal transfer function to minimize non-linear distortion.
2. Prior Arts
In last several years, analog fiber-optics transmission links has found tremendous success in low-bandwidth (<1 GHz) commercial applications such as: cable television (CATV), cellular/personal communication, and fiber-radio access network. One key component in this analog fiber-optics transmission links is a linearized external modulator. Conventional external Mach-Zehnder interferometer (MZI) modulator has sine transfer function which introduces higher-order frequency distortion to the output signal. At low frequency modulation and with fixed radio frequency (RF) signal power level, electrically linearized MZI modulators are very effective, inexpensive, and simple technique to reduce non-linear distortion and thus increase the links' dynamic range. These are reported in technical papers such as Y. Chiu, B. Jalali, S. Garner, and W. Steier, “Broad-Band Electronic Linearizer for Externally Modulated Analog Fiber-Optic Links,” Photonics Technology Letters vol. 11, no. 1 January 1999, pp. 48–50, R. Sadhwani, et al. “Adaptive Electronic Linearization for Fiber,” OFC 2003 vol. 2, pp. 477–479, M. Nazarathy, et al., “Progress in external modulated AM CATV transmission systems”, J. Lightwave Technol., Vol. 11, pp. 82–105, January 1993, and in patents, for example, such as U.S. Pat. No. 5,850,305 (Pidgeon, 1995) and U.S. Pat. No. 5,327,279 (Farina, 1994).
However, as the demands for analog fiber-optic links move toward high-density, high-channel-count commercial CATV and high-bandwidth (>10 GHz) military applications such as; antenna-remoting, beam forming for phased-array radars, and various weapon platforms, advanced external modulator components are needed to meet the more stringent and challenging requirements of these applications. These requirements include both wider bandwidth (DC-20 GHz), and greater link dynamic range (spurious-free dynamic range, SFDR>130 dB-Hz) performance. Furthermore, real-time, adaptive and deployable compensation architectures are necessary due to uncontrollable fluctuation of parameter values in the links/environment. Moreover, all these requirements must be accomplished while minimizing the cost, and reducing the overall complexity of the links system at the same time.
One emerging component that has the potential to meet these requirements is an optically linearized MZI modulators rather than electrically linearized MZI modulators. Other general approaches like (a) directly-modulated lasers have still limited bandwidth (<3 GHz) while (b) optically linearized directional coupler modulators (DCMs) and (c) linearized electro-absorption modulators (EAMs) involve costly fabrication improvements, complicated techniques, and long-term development.
Previous works related to DCMs and EAMs are reported, for example, in technical papers such as: J. Schaffner, J. F. Lam, C. J. Gaeta, G. L. Tangonan, R. L. Joyce, M. L. Farwell and W. S. C. Chang, “Spur-Free Dynamic Range Measurements of a Fiber Optic Link with Traveling Wave Linearized Directional Coupler Modulators,” Photonics Technology Letters, vol. 6, No. 2, February 1994, pp. 273–275, C. H. Bulmer, W. K. Burns, and C. W. Pickett “Linear 0–20 GHz Modulation with a 1×2 Directional Coupler,” Photonics Technology Letters, vol. 3, no. 1, January 1991, pp. 28–30, M. L. Farwell, Zong-Qi Lin, Ed Wooten, and William S. C. Chang “An Electrooptic Intensity Modulator with Improved Linearity” Photonics. Techn. Letters, vol. 3 no. 9, September 1991, pp. 792–795, and G. Kanter, P. Capofreddi, S. Behtash, A. Gandhi, “Electronic Equalization for Extending the Reach of Electro-Absorption Modulator,” OFC 2003, vol. 2, pp. 476–477.
Optically linearized MZI-based modulators offer lesser technological risk, greater reliability and more realistic cost-effective fabrication since (i) the physics/operation of MZI is already well studied, (ii) it uses matured inorganic materials (i.e. Lithium Niobate, semiconductor GaAs, polymers), and (iii) it employs well-established manufacturing technology.
Existing optically linearized MZI-based modulators can be categorized into three broad groups namely: (1) dual-signal MZI modulator, (2) cascaded MZI modulator, and (3) resonator-assisted modulator. The first group uses a single standard MZI modulator with two injected optical signals in the form of (1.1) two polarizations as in the case reported, for example, by L. M. Johnson and H. V. Roussell “Reduction of intermodulation distortion in interferometric optical modulators,” Optics Letters vol. 13, no. 10, October 1988, pp. 928–930, (1.2) two wavelengths as in the case reported by Edward I. Ackerman, “Broad-Band Linearization of a Mach-Zehnder Electro optic Modulator,” IEEE Transactions on Microwave Theory and Techniques, vol. 47 no. 12, 1999, pp. 2271–2279, and U.S. Pat. No. 6,246,500, and (1.3) bi-directional signals as in the case reported by A. Loayssa, M. Alonso, D. Benito and M. J. Garde “Linearization of Electro optic Modulators at Millimeter-wave Frequencies,” IEEE 199912th LEOS Annual Meeting, Proceeding Vol. 1, pp. 275–276.
Its major advantages are simplicity and low-cost. The improvement in spurious-free dynamic range, SFDR is generally in the range of 5 to 15 dB compared with standard non-linearized MZI modulator. Its major disadvantages are design inflexibility, non-optimum performance, and tight tolerance requirements.
The second group consists of two or more standard MZI modulators connected in series arrangements as reported, for example, in William K. Burns, “Linearized Optical Modulator with Fifth Order Correction” Journal of Lightwave Technology, vol. 13 no. 8, 1995, pp. 1724–1727, U.S. Pat. No. 5,148,503 (Skeie, 1992), or in parallel configurations as reported, for example, in S. K. Korotky, R. M. Ridder, “Dual Parallel Modulation Schemes for Low-Distortion Analog Optical Transmission,” IEEE Journal on Sel. Areas in Comm., vol. 8 no. 7, 1990, pp. 1377–1381). It is a generalization of the first group and offers higher SFDR improvement by 10–20 dB (William B. Bridges, and James H. Schaffner, “Distortion in Linearized Electro optic Modulators,” IEEE Transactions on Microwave Theory and Techniques, vo. 43 no. 9, September 1995, pp. 2184–2197).
The major drawbacks of these designs are tight tolerance requirements, huge optical losses, increased power penalty expensive cost since it requires multiple modulators. Furthermore, multiple parameters need precise control that lead to complicated compensation arrangement.
The third group can be referred to as resonator-assisted external modulator. Its basic optical configuration is very similar to the set-up used in optical filtering. X. Xia proposed MZI with ring resonator(s) (RR) coupled in the arm(s) of the interferometer as reported in Xiaobo Xie, Jacob Khurgin, Jin Kang, and Fow-San Chow, “Linearized Mach-Zehnder Intensity Modulator,” IEEE Photonics Technology Letters, vol. 15 no. 4, 2003, pp. 531–533 while N. Rengand employed Gires-Tournois resonator (GTR) in Michelson interferometer (MI) configuration as published in N. Rengand, I. Shpantzer, Ya. Achiam, A. Kaplan, A. Greenbalatt, G. Harston, P. S. Cho, “Novel Design for the Broadband Linearized Optical Intensity Modulator,” IEEE Military Communications Conference. MILCOM 2003. Vol. 2, 13–16, 2003, pp. 1208–1212. Both resonators are operated in the non-resonance region to avoid the bandwidth narrowing. Preliminary studies indicate that 5–15 dB improvement can be achieved. However, it is still limited to less 20 dB improvement and tight tolerance requirements.
In general, all these reported modulators are expensive, have significant optical insertion loss, high power penalty, and only moderate dynamic range improvement. More importantly, all these modulators necessitate complicated optical arrangement to perform dynamic compensation. Thus, there is an urgent need for linearized external modulators that overcome the aforementioned shortcomings in a simple, effective, and economical ways.